Why Get Tested?

When to Get Tested?

With a blood creatinine test as part of a routine health exam or when your healthcare practitioner thinks that you may have kidney damage or that you may be at risk for developing kidney disease

Sample Required?

A blood sample drawn from a vein in your arm; the eGFR is a calculated estimate of the actual glomerular filtration rate and is based on your blood creatinine level along with other variables such as your age, sex, and race, depending on the equation used.

Test Preparation Needed?

This test uses a blood creatinine level to calculate a result. You may be instructed to fast overnight or refrain from eating cooked meat; some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.

How is it used?

The estimated glomerular filtration rate (eGFR) is used to screen for and detect early kidney damage, to help diagnose chronic kidney disease (CKD), and to monitor kidney status. It is a calculation based on the results of a blood creatinine test along with other variables such as age, sex, and race (e.g., African-American, non-African American), depending on the equation used. (For more on the equations, see the "What is being tested?" section.)

The National Kidney Disease Education Program, American Society of Nephrology, and the National Kidney Foundation all recommend that an eGFR be calculated every time a creatinine blood test is done. The creatinine test is ordered frequently as part of a routine comprehensive metabolic panel (CMP) or basic metabolic panel (BMP), or along with a blood urea nitrogen (BUN) test to evaluate the status of a person's kidneys.

Creatinine, along with eGFR, is often used to monitor people with known CKD and those with conditions such as diabetes and high blood pressure (hypertension) that may lead to kidney damage.

Other tests that may be done at the same time to help detect kidney damage and/or evaluate kidney function are:

Urinalysis–may be used to help detect signs of kidney damage, such as the presence of blood or casts in the urine

When is it ordered?

A creatinine test and eGFR may be ordered when a healthcare practitioner wants to evaluate a person's kidney function as part of a health checkup or if kidney disease is suspected. Signs and symptoms of kidney disease may include:

Swelling or puffiness, particularly around the eyes or in the face, wrists, abdomen, thighs, or ankles

Urine that is foamy, bloody, or coffee-colored

A decrease in the amount of urine

Problems urinating, such as a burning feeling or abnormal discharge during urination, or a change in the frequency of urination, especially at night

Mid-back pain (flank), below the ribs, near where the kidneys are located

High blood pressure (hypertension)

As kidney disease worsens, symptoms may include:

Urinating more or less often

Feeling itchy

Tiredness, loss of concentration

Loss of appetite, nausea and/or vomiting

Swelling and/or numbness in hands and feet

Darkened skin

Muscle cramps

An eGFR may be repeated if the initial result is abnormal to see if it persists.

The test is usually ordered periodically when a person has a chronic kidney disease (CKD) or a condition such as diabetes or hypertension that is associated with an increased risk of kidney damage.

What does the test result mean?

Estimated GFR results are reported as milliliters/minute/1.73m2 (mL/min/1.73m2). Because some laboratories do not collect information on a patient's race when the sample is collected for testing, they may report calculated results for both African Americans and non-African Americans. The healthcare practitioner uses the result that applies to the particular patient in order to interpret the results correctly.

A normal eGFR for adults is greater than 90 mL/min/1.73m2, according to the National Kidney Foundation. (Because the calculation works best for estimating reduced kidney function, actual numbers are only reported once values are less than 60 mL/min/1.73m2).

An eGFR below 60 mL/min/1.73m2 suggests that some kidney damage has occurred. The test may be repeated to see if the abnormal result persists. Chronic kidney disease is diagnosed when a person has an eGFR less than 60 mL/min/1.73m2 for more than three months.

A person may have some kidney damage even with an eGFR greater than 90 mL/min/1.73m2. Other evidence, such as increased urine albumin, may indicate some degree of kidney damage. Thus, a person's eGFR should be interpreted in relation to the person's clinical history and presenting conditions.

The following table summarizes estimated GFR and the stages of kidney damage:

Kidney damage stage

description

estimated gfr (ML/MIN/1.73M2)

other findings

1

Normal or minimal kidney damage with normal GFR

90+

Protein or albumin in urine may be high, cells or casts rarely seen in urine (see Urinalysis)

2

Mild decrease in GFR

60-89

Protein or albumin in urine may be high, cells or casts rarely seen in urine

3

Moderate decrease in GFR

30-59

4

Severe decrease in GFR

15-29

5

Kidney failure

<15

Is there anything else I should know?

Another method of evaluating kidney function and potentially estimating GFR involves the measurement of the blood level of Cystatin C. There is increasing interest in the use of this test for these purposes and several studies have been performed comparing calculations of eGFR using creatinine, cystatin C, or both. According to the National Kidney Foundation (NKF), two meta-analyses concluded that cystatin C is superior to creatinine as a marker of kidney function. The NKF also states that a formula for calculating eGFR that includes both blood creatinine and cystatin C values may improve that estimate (see Common Questions #7).

The creatinine clearance test also provides an estimate of kidney function and of the actual GFR. However, in addition to the serum creatinine, this test requires a timed urine collection (24 hours) for urine creatinine measurement in order to compare blood and urine creatinine concentrations and to calculate the clearance.

The actual amount of creatinine that a person produces and excretes is affected by their muscle mass and by the amount of protein in their diet. Men tend to have higher creatinine levels than women and children.

A person's GFR decreases with age and some illnesses and usually increases during pregnancy.

A slightly different equation should be used to calculate the eGFR for those under the age of 18 (see Common Questions #2). The eGFR equations are not valid for those who are 70 years of age or older because muscle mass normally decreases with age.

An eGFR may not be as useful for those who differ from normal creatinine concentrations. This may include people who have significantly more muscle (such as a body builder) or less muscle (such as a muscle-wasting disease) than the norm, those who are extremely obese, malnourished, follow a strict vegetarian diet, ingest little protein, or who take creatine dietary supplements.

The eGFR may also be affected by a variety of drugs, such as gentamicin, cisplatin, and cefoxitin that increase creatinine levels, and by any condition that decreases blood flow to the kidneys.

The calculation for eGFR is intended to be used when kidney function and creatinine production are stable. If a creatinine level is measured when the kidney function is changing rapidly, such as with acute kidney failure, then it will not give a useful estimate of the filtration rate.

What is being tested?

Glomerular filtration rate (GFR) is a measure of the function of the kidneys. This test measures the level of creatinine in the blood and uses the result in a formula to calculate a number that reflects how well the kidneys are functioning, called the estimated GFR or eGFR.

Glomeruli are tiny filters in the kidneys that allow waste products to be removed from the blood, while preventing the loss of important constituents, including proteins and blood cells. Every day, healthy kidneys filter about 200 quarts of blood and produce about 2 quarts of urine. The GFR refers to the amount of blood that is filtered by the glomeruli per minute. As a person's kidney function declines due to damage or disease, the filtration rate decreases and waste products begin to accumulate in the blood.

chronic kidney disease (CKD) is associated with a decrease in kidney function that is often progressive. CKD can be seen with a variety of conditions, including diabetes and high blood pressure. Early detection of kidney dysfunction can help to minimize the damage. This is important as symptoms of kidney disease may not be noticeable until as much as 30-40% of kidney function is lost.

Measuring glomerular filtration rate directly is considered the most accurate way to detect changes in kidney status, but measuring the GFR directly is complicated, requires experienced personnel, and is typically performed only in research settings and transplant centers. Because of this, the estimated GFR is usually used.

The eGFR is a calculation based on a serum creatinine test. Creatinine is a muscle waste product that is filtered from the blood by the kidneys and released into the urine at a relatively steady rate. When kidney function decreases, less creatinine is eliminated and concentrations increase in the blood. With the creatinine test, a reasonable estimate of the actual GFR can be determined.

Different equations may be used to calculate eGFR. The following two are most common and require a person's blood creatinine result, age, and assigned values based upon sex and race.

How is the sample collected for testing?

A blood sample is taken by needle from a vein in the arm. Depending on the formula used, a person's age, sex, race, height, and weight may also be needed.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

This test uses a blood creatinine level to calculate a result. You may be instructed to fast overnight or refrain from eating cooked meat; some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.

The best method for directly determining the GFR is a procedure called an "inulin clearance." It involves introducing a fluid containing the marker molecule inulin (NOT insulin) into your veins (IV - intravenous infusion) and then collecting timed urines over a period of hours. The urine volumes are noted and the inulin in each sample is measured to allow determination of the GFR. This test and other methods of determining GFR, such as those that use radioactive markers, are not routinely ordered and are primarily performed in research settings.

Could I calculate my own eGFR?

If you have had a recent Creatinine or cystatin C measurement, you can calculate the eGFR by using one of the calculators for people 19 years of age or older on the National Kidney Foundation (NKF) web site. If you have questions about the interpretation of your results, it is best to consult with your healthcare provider. For children and teens younger than 19, see the pediatric eGFR calculator on the NKF website.

Is an eGFR always calculated when a creatinine test is ordered?

This practice is recommended and has been adopted by most laboratories. The eGFR can always be calculated at your healthcare provider's request.

Why might my healthcare provider repeat my eGFR test?

Besides for periodic monitoring, the eGFR might be repeated if your healthcare provider feels that a temporary condition may be affecting your results.

What other findings might suggest kidney dysfunction?

Diabetics and others at risk for developing kidney disease may be monitored for small amounts of albumin in their urine by performing a urine albumin test. The presence of albumin and other plasma proteins as well as blood in the urine can all be signs of potential kidney damage.

Why are factors such as age, sex, and race used in eGFR calculations?

This is because these factors are known to affect the level of creatinine in the blood. Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. According to the National Kidney Foundation, men tend to have more muscle mass than women so their creatinine levels tend to be higher. African Americans have a higher average muscle mass and generate more creatinine. As we age, we lose muscle mass and thus have lower blood creatinine levels for the same kidney function the older we get.

Besides measuring blood creatinine, are there other ways of estimating GFR?

Yes, a set of new Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) calculators was published in 2012. One uses the measure of a substance called cystatin C instead of creatinine to calculate eGFR while the other uses measures of both blood creatinine and cystatin C to estimate GFR. Cystatin C is a relatively small protein that is produced throughout the body by all cells that contain a nucleus and is found in a variety of body fluids, including the blood. It is produced, filtered from the blood by the kidneys, and broken down at a constant rate. Unlike creatinine, cystatin C is not significantly affected by muscle mass.

According to the National Kidney Foundation, the newer CKD-EPI equations may be useful for assessing kidney function in people who have differences in diet, such as vegans, very high or very low muscle mass (e.g., body builders or those with muscle-wasting diseases), or for those who have changing muscle mass. They also may be useful for identifying people diagnosed with Chronic kidney disease who have the highest risk of complications.

Elsewhere On The Web

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used. To access online sources, copy and paste the URL into your browser.

Descriptions of clinical laboratory tests were originally prepared for use on Lab Tests Online, an award-winning patient education website on clinical laboratory testing. Lab Tests Online is produced by the American Association for Clinical Chemistry (AACC), a global scientific and medical professional organization dedicated to clinical laboratory science and its application to healthcare. The Lab Tests Online website is developed in collaboration with other laboratory professional societies and is funded in part through corporate sponsorships.